Wavelength division multiplexing (WDM) of optical beams is presently being used to increase the rate of transmission of information through an optical fiber. These multi-wavelength optical beams provide information by a plurality of signal components, also referred to as optical channels. Each channel is defined by a unique wavelength of light that are multiplexed together and transmitted through a communication link of an optical network. In order to detect any problems or potential problems of the optical network and to provide feedback to active components within the network, a plurality of status monitors may be located throughout the network to provide information indicative of the health or condition of the optical beams transiting through the network. This information includes the amplitude and signal to noise ratio of each of the channels of the optical beams.
The status monitor also may be used to compensate for gain variations throughout the optical network. Current wavelength division multiplexed (WDM) intercity communications links, as shown in FIG. 1, require a number of amplifiers along the link length to compensate for fiber loss. The intercity links can run approximately 600 meters. There is currently a need for an amplifier every 80 to 120 kilometers in these links.
Dynamic operating conditions, such as the addition and subtraction of the channels, and nonuniform gain characteristics of the amplifiers result in gain variations between the channels each time the optical beam is amplified. The prior art of FIG. 1 does not provide compensation for these conditions.
FIG. 1 shows a graphical representation of a typical communication link 2 of an optical network. A plurality of light generators (LG) 3 provide respective component signals 12 of select wavelengths that are combined by a multiplexer 4 to produce the multi-wavelength optical beam 14. Before the component signals are multiplexed, a plurality of pre-emphasis devices (PE) 5 amplify selectively each of the respective component signals 12. As mention hereinbefore, a plurality of amplifiers 20 amplify the optical beam 14 to compensate for fiber loss as the beam passes therethrough. At the receiver end of the communication link, the signal components 12 of the optical beam are then separated by a demultiplexer 6 and provided to a corresponding receiver (R) 7.
The prior art 2 does not provide any compensation to overcome the nonuniform gain of the each amplifier 20. As shown in each of the plots 14, which are representative of the amplitude of the output power of each channel 12, the differential of the output power of each of the channels increase after each gain stage 20. The output power of each channel of the communication link at 8, therefore, are not equal. The only compensation provided by the prior art is adjustment of the pre-emphasis devices 5 for amplifying each channel 12 to ensure adequate signal level and signal-to-noise is achieved.
A device that provides feedback of the channels is a Fabry-Perot spectrum analyzer. This spectrum analyzer includes two pieces of an optical fiber that are coated to become a broadband high reflector and are laterally-spaced and aligned with each other. The distance between the ends of the fibers are varied to determined the power spectrum of each optical signal. The spectrum analyzer and associated electronic control is very costly. These devices are bulky and unreliable due to its moving parts. In addition, calibration of these devices are difficult to maintain due to drift and hysterisis. These spectrum analyzers also do not provide simultaneous monitoring of each channel, but scans through each channel of the multi-wavelength optical beam to provide a serial output.
Accordingly, it is the principal object of this invention to provide an inexpensive, compact status monitor that provides signals indicative of the condition of each channel of an optical beam.
It is another object of this invention to provide a status monitor that does not require calibration which permits the status monitor to be interchangeable throughout the optical network.
It is a further object of this invention to provide a status monitor that generates simultaneously output signals of each channel that are indicative of the condition of each signal.